What is Docker Compose

Docker Compose is a powerful tool designed to manage multiple Docker containers as a single, cohesive application. It simplifies the process of building, running, and managing complex applications using a single configuration file.


Installation

Before getting started with Docker Compose, ensure that both Docker and Docker Compose are installed on your system. You can find installation instructions on the official Docker website. Docker Compose can be downloaded from GitHub or installed using a package manager.


The Docker Compose YAML File

At the core of Docker Compose lies the YAML configuration file
(docker-compose.yml), where you define the configuration of your services. Here’s an overview:

version: "3.8" # Docker Compose syntax version

services: # List of services to run
  service1: # Service name
    image: image1:tag # Docker image to use
    ports: # Port forwarding (optional)
      - "8000:8000"
    environment: # Environment variables (optional)
      - ENV_VAR=value

Common Commands

  • docker-compose up: Builds, (re)creates, and starts all services.
  • docker-compose down: Stops and removes all services along with their associated networks and volumes.
  • docker-compose ps: Shows the status of all services.
  • docker-compose logs: Displays the logs of all services.
  • docker-compose exec <service> <command>: Executes a command in a running service.

Pro Tips

Network Configuration

You can define custom networks in your Docker Compose file to ensure isolated communication between services for example:

version: "3"
services:
  web:
    image: nginx
    networks:
      - my-network

  db:
    image: postgres
    networks:
      - my-network

networks:
  my-network:
    driver: bridge

This example demonstrates how to define a custom network named my-network in a Docker Compose file, and how to assign services (web and db) to this network to ensure isolated communication between them.


Volumes

Use volumes to make data persistent even after containers are stopped for example:

version: "3"
services:
  web:
    image: nginx
    volumes:
      - my-volume:/usr/share/nginx/html

volumes:
  my-volume:

In this example, a volume named my-volume is defined, and it’s mounted to the /usr/share/nginx/html directory within the web service container. This ensures that data stored in this directory persists even if the container is stopped or removed.


Restart Policy

Customize the restart policy for each service to determine how it responds to crashes for example:

version: "3"
services:
  web:
    image: nginx
    restart: always

  backend:
    image: my-backend
    restart: on-failure

In this example, the web service has a restart policy set to always, which means Docker will always attempt to restart the container if it stops for any reason. On the other hand, the backend service has a restart policy set to on-failure, indicating that Docker will only restart the container if it exits with a non-zero status code (i.e., if it fails).


Secrets and Configurations

Use Docker Secrets or external configuration providers to manage sensitive information like passwords for example:

version: "3.1"
services:
  db:
    image: postgres
    environment:
      POSTGRES_PASSWORD_FILE: /run/secrets/db_password
    secrets:
      - db_password

secrets:
  db_password:
    file: ./db_password.txt

In this example, a Docker secret named db_password is created, which reads the password from a file named db_password.txt. This secret is then mounted into the db service container at the path /run/secrets/db_password, allowing the PostgreSQL container to access the password securely without exposing it in the Docker Compose file.


Advanced Features

Docker Compose Overrides

Use multiple YAML files to define different configurations for different environments (e.g., development, staging, production) for example:

Suppose you have a base docker-compose.yml file:

version: "3"
services:
  web:
    image: nginx
    ports:
      - "8080:80"
    volumes:
      - ./html:/usr/share/nginx/html

And then, for the development environment, you can have a docker-compose.override.yml file:

version: "3"
services:
  web:
    ports:
      - "8080:80"
    environment:
      DEBUG: "true"
    volumes:
      - ./html:/usr/share/nginx/html

In this example, the docker-compose.override.yml file is used to override specific settings from the base docker-compose.yml file for the development environment. Here, the web service’s ports and environment are modified to suit the development environment while keeping the volume mapping unchanged.

This example demonstrates how you can use Docker Compose override files to customize configurations for different environments while keeping a common base configuration in the docker-compose.yml file.


Service Scaling

Easily scale services up or down to meet demand for example:

Suppose you have a simple docker-compose.yml file defining a service:

version: "3"
services:
  web:
    image: nginx

To scale the web service to run multiple containers, you can use the
docker-compose command:

docker-compose up -d --scale web=3

This command instructs Docker Compose to start three instances of the web service. You can adjust the number according to your needs. Scaling services allows you to distribute the workload across multiple containers, improving performance and resilience.

In this example, the docker-compose up -d --scale web=3 command scales the web service to run three instances of the nginx container. This demonstrates how Docker Compose enables easy scaling of services to meet varying demand.


Conclusion

Docker Compose is a game-changer for streamlining the development, testing, and deployment of Docker-based applications. Explore its capabilities and witness how it enhances your workflow seamlessly!

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Last updated 01 Sep 2024, 10:22 CEST . history